Taking place in a science-futuristic setting (where all of the characters speak alienese by sticking out their lower jaws and talking), Robiya Hime sends Luffytaro to find Mugiwaraman (not knowing they are one and the same) and stop the Universal Association of Darkness from invading the moon. Along the way, Luffytaro becomes Mugiwaraman and saves Planet Namitan by deflecting an octopus-shaped asteroid. When Luffytaro arrives at the villains' base, agent Choppasmith tells him they have gone to invade the moon. But by the time anyone gets there, it's already been destroyed by that octopus-shaped asteroid.
Robiya Hime: Queen of the Moon, played by Nico Robin. She sends Luffytaro to find Mugiwaraman and stop the villains.
Luffytaro: Lunar Special Defense Force Soldier, played by Monkey D. Luffy. Luffytaro is secretly the Super Space Hero of Justice, Mugiwaraman, a parody of Ultraman. Mugiwaraman apparently has the same rubber body as Luffy in the normal story, except he replaces "Gomu Gomu" with "Mugi Mugi". The only technique seen is the Mugi Mugi no Fusen. If he stays transformed for over five minutes, he will be unable to eat meat for the rest of his life.
Sanvader: The Martian Martial Artist of Darkness, played by Sanji. One of the Universal Association of Darkness' leaders, he wants to conquer the moon. He's a parody of Darth Vader from Star Wars. It also may be simply a pun on "Invader".
Zorodadener: Venusian Swordsman of Darkness, played by Roronoa Zoro. One of the Universal Association of Darkness' leaders, he wants to conquer the moon.
Usoshakeen: The Big Space Robot built by Sanvader, played by Usopp. Sanvader claims his sniping accuracy speed is the greatest in all space.
Frangashahn: The Big Space Robot built by Zorodadener. Played by Franky. Zorodadener claims he is the strongest robot in all space.
Namitanian: An Alien Humanoid, played by Nami. Her homeplanet Namitan is in danger of being destroyed by an octopus-shaped asteroid, so she sends an SOS out. She is based off the Ultraman species "Baltan". Her appearance highly resembles the one of a "Space Pirate" from the sci-fi video game series Metroid.